Caption: Iodine (I2) subliming and crystallizing (sparkles) in a heated flask. Iodine is a black solid at room temperature that consists of weakly bonded units (molecules) of two iodine atoms. Gentle heating will result in these bonds being broken and purple iodine gas forming. The solid to vapour transition is known as sublimation. The vapour recrystallizes on the cool surfaces of the flask, and is purified in the process. Iodine's quoted boiling point (184 degrees Celsius) refers to the point at which the pressure exerted by the iodine vapour is equal to the atmospheric pressure. Iodine, found in seaweed and sea water, is an essential part of the diet.